1. Field of the Invention
The present invention relates to optical communications, and more particularly to devices and methods for manipulating a multi-frequency input signal, such as a WDM or DWDM signal, to perform a variety of optical signaling processes.
2. Background of the Invention
Fiber optic cable is widely used for data transmission and other telecommunication applications. The relatively high cost of installing new fiber optic cable presents a barrier to increased carrying capacity, however, so it is highly desirable to increase the data carrying capabilities of existing fiber. Wavelength division multiplexing (WDM) enables different wavelengths to be carried over a common fiber optic waveguide. WDM can separate the fiber bandwidth into multiple discrete channels through a technique referred to as dense wavelength division multiplexing (DWDM). This provides a relatively low cost and proven method of substantially increasing long-haul telecommunication capacity over existing fiber optic transmission lines. Techniques and devices are required, however, for performing a variety of processing procedures on the input signals, such procedures including: multiplexing and de-multiplexing the different discrete carrier wavelengths, adding new signals to and dropping existing signals from the multiplexed signal, specialized coding processing such as code division multiple access, precisely controlling the wavelength of laser sources throughout the WDM communications process, improving signal quality (filtering) by reducing crosstalk and signal power loss as channel frequency spacings decrease, and channel interleaving and de-interleaving, among others.
Multiplexing and De-multiplexing. As part of the mux/demux process, individual optical signals should be combined onto a common fiber optic waveguide and then later separated again into the individual signals or channels at the opposite end of the fiber optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength bands) from a broad spectral source is of growing importance to the fiber optic telecommunications field and other fields employing optical instruments.
Devices that assemble multiple tightly spaced carrier wavelengths within a single fiber are called multiplexers. Devices that separate the carrier wavelengths at the receiving end of a fiber are called de-multiplexers or channelizers. WDM channelizers known in the prior art include Fabry-Perot interferometers, Lummer-Gehrcke interferometers, Virtually Imaged Phased Array (VIPA) generators, Thin film interference filters, arrayed waveguide gratings, and optical fiber Bragg gratings.
The Fabry-Perot interferometer is a known device for resolving light into its component frequencies, or equivalently, its component wavelengths. FIG. 1 illustrates one example of a prior art Fabry-Perot interferometer. It includes two parallel partially reflective mirrors 70 and 71. The mirrors are separated by a cavity 72, which might be an air space or alternatively, a solid transparent material. Light from a spectrally broadband source, i.e., a laser, is input at plane 75. In particular, a multi-spectral light ray input from point P1 entering through the partially reflective mirror 70 at an angle θ undergoes multiple reflections between mirrors 70 and 71. The emerging light rays 76, having a common wavelength λ, interfere constructively along a circular locus P2 in the output plane 77 where an appropriate detector might be positioned. The condition for constructive interference that relates a particular angle θ and a particular wavelength λ is given by the formula2d cos θ=mλwhere d is the separation of the reflecting surfaces and m is an integer known as the order parameter. The Fabry-Perot etalon thereby separates the component frequencies of the input light by using multiple beam reflections and interferences. It is apparent from the equation above that the output light pattern of the system, i.e., the interference fringes, in the case of a diverging input beam, is a set of concentric circular rings. One ring is present for each wavelength component of the input light for each integer m, with the diameter of each ring being proportional to the corresponding light frequency.
The Fabry-Perot interferometer is not well suited for use as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a set of rings. Multiple wavelengths produce nested sets of concentric rings. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the interferometer has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the beam can be fanned over a narrow range of angles to produce only a single-order output (e.g., m=+1) for each wavelength of interest. This fanning makes it easy to concentrate the output light at multiple detector points or fibers, but there is inherently high loss. The throughput efficiency can be no greater than 1/N, where N is the number of wavelength components to be separated.
FIG. 2 illustrates an example of a prior art Lummer-Gehrcke interferometer. The illustrated interferometer comprises an uncoated glass plate P and a prism R for coupling a beam of light B into the plate. Internally, the plate is highly reflective at internal incidence angles that approach the critical angle. The internal incidence angle controls the reflectivity of the surfaces. The output of the illustrated Lummer-Gehrcke interferometer is a series of multiple reflected beams b1, b2, b3 that have a frequency-dependent phase shift from beam to beam and that are focused at the output plane S by a lens L. The interference fringes that are formed at the output plane in the case of a diverging input beam and a particular wavelength λ are a family of hyperbolae H near the center of the output plane. Each wavelength component of the input beam gives rise to a unique set of hyperbolic fringes.
The Lummer-Gehrcke interferometer relies upon a glass plate that is uncoated. However, the absence of a surface coating means that it is not possible to tailor the fringe intensity profile. This makes the Lummer-Gehrcke interferometer impractical for use in WDM applications in which the fringe profile controls the channel filter shape.
The Lummer-Gehrcke interferometer also is not well suited for use as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a family of hyperbolae. Multiple wavelengths produce nested sets of hyperbolae. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the interferometer has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the output pattern for a given wavelength is a set of focused spots corresponding to multiple interference orders. Again, it is difficult to collect this light efficiently, and there is generally an inherent loss. The throughput efficiency can be no greater than 1/N, where N is the number of focused spots per wavelength.
FIG. 3 illustrates an example of a prior art virtually imaged phased array (commonly referred to as a VIPA). The VIPA illustrated in FIG. 3 includes a rectangular glass plate 100 that has a 100% reflective coating 101 on a first side and a partially reflective coating 102 on an opposing side. Light enters the plate 100 below the reflective coating 101 in the form of a focused line source 105 produced by a cylinder lens 106.
FIG. 4 illustrates an operational side view of the VIPA. Input light rays 107 and 110 represent the boundaries of the line-focused input beam. The lens 106 focuses the input rays at the point 111, after which the rays diverge as the beam propagates. The focused input rays 107 and 110 are partially reflected by the coating 102 and then totally reflected by the coating 101. This reflection produces a virtual image of point 111 at location 112. The reflective process is continued, producing additional receding virtual images at locations 115 and 116. This process produces virtual images of the line source that recede away from the input side of the glass plate (i.e., to the left of element 100 in FIG. 3) and that are distributed in the y direction.
FIG. 5 illustrates an example of the optical distribution of the diverging light beams at the exit surface of the glass plate. The numbered circles 117, 120 and 121 are intended to call the reader's attention to the areas of interest on the coated surface 102. The circles represent the size of the light beams exiting the plate. The line focused input is illustrated at point 111, the twice reflected light that has diverged due to propagation is illustrated at circle 117, the four-times reflected light that has diverged even more is illustrated at circle 120, and the six-times reflected light that has diverged even more is illustrated at circle 121. In the example illustrated in FIG. 5, after more than six reflections the diverging light beams overlap and blend into an interference pattern.
As shown in FIG. 5, each of the successive beams 117, 120 and 121 that exits the surface 102 of the VIPA plate 100 appears to originate from the line source images 112, 115 and 116, respectively, as shown in FIG. 4. The light from these images diverges as the light propagates inside the glass plate 100. Part of the light from each image exits the plate through the partially reflective coating 102. The exiting beams interfere with each other. The interference pattern is collected by a lens 122 and focused onto a 1-dimensional detector array 125 (FIG. 3).
In the illustrated VIPA the beams diverge and overlap at the partially reflective surface 102. This overlap effect makes weighting the individual virtual sources possible only in an area-average sense, thus limiting the degree to which the channel filter shape can be tailored.
The VIPA requires a line-focused input. The line-focused input means that the VIPA may provide a relatively compact device for coarse channelization (i.e., wide channel spacing on the order of one hundred GHz). However, the line-focused input makes the VIPA impractical for fine channelization (i.e., narrow channel spacing on the order of one GHz) due to the fact that a thicker plate is needed, which would result in excessive beam divergence and overlap at the exit surface.
Thin film interference filters require a different coating design to separate each wavelength component of an input beam. A stacked thin-film multiplexer is taught in U.S. Pat. No. 6,111,474. Since the interference filters produced by thin film coatings tend to have relatively wide passbands, they cannot achieve high resolution (twenty-five GHz or finer). In addition, stacking thin-film filters such as the device in U.S. Pat. No. 6,111,474 produces loss that effectively limits the number of channels.
In the classical diffraction grating, as used for WDM channelization applications, the dispersive element is a grating embedded in a monoblock of silica. The input optical fibers may be directly fixed to the block. The grating may comprise several tens of grooves to several thousands of grooves per millimeter. The grooves may be fabricated, for example, by etching with a diamond tool or by holographic photo-exposure. The grating diffracts incident light in a direction related to the wavelength of the light, the groove spacing, and the incidence angle. Consequently, an incident beam comprising several wavelength components will be angularly separated into different directions. Conversely, a reverse mode of operation is possible in which several beams of different wavelengths coming from different directions may be combined (multiplexed) into the same output direction.
Diffraction gratings of reasonable size do not have sufficient resolution for HWDM. For example, for a channel separation of 1 GHz, a grating would have to be significantly longer than twelve inches to achieve an adequate channel filter shape. They also have high optical insertion loss, making them inefficient for use in high-resolution WDM systems.
Diffraction gratings tend to produce undesirable polarization effects. The diffraction efficiency depends on the polarization of the incident beam. For a given wavelength, this effect increases when the groove spacing decreases. Typically this effect is small when the groove spacing is at least ten times larger than the wavelength, but the effect becomes important when the groove spacing is reduced to a few wavelengths in order to achieve higher angular dispersions.
The prior art arrayed waveguide grating (AWG) is an integrated-optic passive delay line device that performs channelization. The AWG is designed to increase the resolving power, i.e. the fine splitting of the wavelengths, over that obtainable with classical diffraction gratings. Around 1990, AWGs were first proposed both by Takahashi and others in Japan, and by Dragone and others in the United States. Arrayed waveguides gratings increase the optical path difference between the diffracting elements by using a waveguide structure that is equivalent to the well-known Michelson echelon gratings in classical optics. AWGs have the inherent disadvantage of a much smaller free spectral range that limits the total number of channels and increases the near-end crosstalk that affects bidirectionality. It is difficult to achieve resolution better than fifty GHz using an AWG. AWG devices capable of one GHz resolution would be physically large, expensive, and have very high loss.
A fiber grating is made by recording a Bragg grating inside the core of a single-mode fiber, which is made photosensitive by doping it with germanium, for example. This grating may be used as a narrowband filter. It is important to use one grating per wavelength channel, so there is an inherent limitation on the number of channels that can be de-multiplexed with such devices due to the shear bulk of the resulting system. A primary disadvantage of a fiber grating is that it can reflect only one wavelength. An N channel system therefore requires N fiber gratings.
Optical Add-Drop Multiplexers. Techniques and devices are known for multiplexing the discrete wavelengths in DWDM transmission systems, i.e., for combining individual optical signals onto a common fiber optic waveguide. Then, the optical signals are separated again into the individual signals or channels at the opposite end of the fiber optic cable. The ability to effectively combine and then separate individual wavelengths (or wavelength bands) from a broad spectral source is of significant importance to the fiber optic telecommunications field. Similarly, this technique is important in many other fields employing optical networking devices.
As fiber optic transmissions enter and leave metropolitan and local area networks (LANs), each data-carrying wavelength is usually switched through various points along the fiber optic network. These points are known as “nodes.” At node locations, optical signals can be forwarded to the next node or “dropped” towards their final destination via the best possible path. The best possible path may be determined by such factors as distance, cost, and the reliability of specific routes. In addition, specific data-carrying wavelengths may be recombined or “added” to the multiplexed optical signal at node sites. The devices that perform these functions in DWDM network systems are called add/drop multiplexers (ADMs).
A conventional way to drop a data signal from a DWDM fiber is to de-multiplex the signal into its constituent wavelengths. Next, the light is detected using a photodetector, thus converting the signals to an electronic form (OE conversion). The electronic signal is switched and/or routed, as appropriate. The remaining signals are converted back to an optical signal (EO conversion). The optical signal is then sent down the proper fiber. During this last step, a signal can be added to the remaining signals. Such OE and EO conversion operations are both protocol and data rate dependent. These operations also require inflexible devices that are costly and difficult to upgrade as system capacity demand is increased.
Optical add/drop multiplexers (OADMs) have several significant advantages. First, OADMs cost less because they eliminate the need for much of the expensive high-speed electronics in conventional devices. Second, OADMs require smaller packaging because removing the electrical conversion step results in a reduced component count within the switches. Finally, optical devices are relatively future-proof because the optics can accommodate any bit-rate, whereas electrical devices should always be customized for the bit-rate and protocol of the signals.
Optical add/drop systems are comprised of two major subsystems. The first subsystem is the de-multiplexing and multiplexing subsystem for selecting and recombining the appropriate wavelength. The second subsystem is the add/drop apparatus for routing the wavelength to the desired optical fiber output. Existing techniques for wavelength separation from a multiplexed signal using optical architectures include thin film bandpass filters, Fabry-Perot filters, fiber Bragg or diffraction grating filters, and polarization controllers. Each of these optical filtering methods may have different forms.
Thin film bandpass filters have traditionally been used in OADM devices to select single wavelengths from a multi-channel optical signal. Although such filters have good channel isolation, they tend to exhibit a transmission light loss of approximately 10%. Such filters are also highly temperature-sensitive. Further, they often operate in only one direction. In addition, such filters are limited to a single, fixed wavelength. Thus, to construct a multi-channel OADM device, multiple filters must be combined. This results in increased complexity, optical loss, and cost.
In U.S. Pat. No. 5,751,456, Koonen disclosed an example of a solution to some of these issues wherein a narrow-bandpass Fabry-Perot filter was utilized in a bi-directional OADM. As Fabry-Perot filters can have a bandpass of 1-2 nm or less, they can provide better isolation and lower loss factors than other thin film interference filters. FIG. 6 illustrates an example of the Koonen prior art. The device illustrated in FIG. 6 is limited in that it can add/drop only a single wavelength. As illustrated in this example, a circulator 127 is used to pass four wavelengths λ1-λ4 to a Fabry-Perot filter 130. Filter 130 selects one wavelength λ1 for continuation on to a receiver 131. The remaining wavelengths λ2-λ4 are reflected by the filter 130 back to circulator 127. A transmitter 132 sends a new wavelength λ1′ to the filter 130. The new wavelength λ1′ is multiplexed with the original wavelengths λ2-λ4. The resulting wavelength is returned to the circulator 127 for continuation
The issue of such interference filter-based ADM devices being fixed in nature has been addressed in the prior art with the invention of “tunable” filters. Tunable filters can be selectively tuned to different wavelengths within a multi-channel optical signal. However, tuning thin-film optical filters requires that either the incident optical beam be repositioned with respect to the filter surface or that the filter itself be repositioned with respect to the input beam. Both scenarios require mechanical movement of components such as actuators or stepper motors. The mechanical movement of these components makes these OADM devices active in nature. This results in increased complexity and cost.
FIG. 7 illustrates an example of a prior art tunable filter as disclosed in U.S. Pat. No. 6,292,299. FIG. 7 illustrates the mechanical nature of selecting a single wavelength. FIG. 7 also illustrates the potential complexity of matching the add/drop wavelengths to output fibers. An electronic controller 135 controls an x-z filter positioner 138 to direct an optical filter 136 to move in the x and z directions to a specific location where a single wavelength from an incoming fiber 137 is intercepted. Once selected, the wavelength is passed or dropped to a fiber 140. The unselected wavelengths are reflected to continue on a fiber 141. A wavelength can be added from fiber 140 at the same time. As can be seen from the example illustrated in FIG. 7, the electronic controller should be mechanically manipulated to select a single wavelength.
Diffraction gratings and fiber Bragg grating filters (FBGs) offer alternative means of selecting and isolating single wavelengths from a multi-channel input beam in OADM devices. Diffraction gratings can be used in an OADM device to separate an input beam into its components in one direction, and recombine the wavelengths in the reverse direction. However, with diffraction grating systems, the component count can rise rapidly. Lenses, collimators, and focusing optics are required to refine, direct, and couple the light beams into fibers.
Because FBGs are constructed from optical fibers, rather than individual thin-film filter substrates, they allow for all-fiber systems to be constructed. Fiber Bragg grating systems offer high levels of selectivity. However, they are limited in that several fiber gratings must be combined, along with optical circulators, in order to handle a multiplexed optical signal with a high channel count. The result can be a very large device with a high component count, increased complexity, and a higher cost. In addition, the combination or cascading of multiple-fiber Bragg gratings can significantly reduce signal strength as the insertion loss of multiple devices is compounded throughout the system.
A recent development in the area of wavelength selectivity and separation of multiplexed optical signals has been the utilization of polarization controllers. As disclosed by U.S. Pat. No. 6,285,478, polarization-controlling elements can also be used within OADM devices to separate a multi-channel WDM input signal into odd and even channels. This is done, for example, by splitting the signal into its vertically and horizontally polarized components. When combined with birefringent beam displacing optics, the separated signals can then be directed to appropriate output paths. This method provides an add/drop device that can accommodate the high channel counts and narrow channel spacing of current DWDM networks, where channels are separated by 50 GHz or less. This channel-separation technique is expandable and can adapt to increasing channel counts. However, this technique is subject to very high optical component count. Included in the optical component count are multiple polarization controllers, birefringent elements and beam splitters. These components are required to manipulate dense multiplexed signals. Assembling and aligning these optical components within a device can be extremely expensive. This is particularly true when high levels of precision are required. FIG. 8 illustrates an example of the prior art requirement for a large number of components to separate eight channels.
With the continued development of WDM fiber optic systems, it is becoming increasingly important to control the direction of wavelengths to desired output ports (i.e., routers). It is likewise important to permit a new signal to replace an existing signal at a specific wavelength (i.e., add/drop) using optical systems. Furthermore, since the development of DWDM sends hundreds and even thousands of wavelengths through a fiber, the ability to selectively control a single or several wavelengths without affecting the other wavelengths is very important. This ability is important because the optical to electrical to optical conversion process is expensive and uses significant power as well as space. In particular, optical add/drops are important components in WDM regional-access ring or star networks to provide broadband access to users.
Prior art optical subsystems that perform add/drop functions include mirrors and micro-electro-mechanical systems (MEMS) using movable and fixed micro-mirrors and etalons. These micro-mirrors are a reconfigurable switching matrix capable of directing the output of wavelengths in multiple directions.
It is also known in the art to use a tunable optical add/drop that employs an optical filter device, such as a multi-layer dielectric wedge filter. This technique is successful using tunable Mach-Zehnder interferometers, acoustic tuning filters, tunable thin film interference filters, tunable Fabry-Perot etalons, and tunable Fabry-Perot interferometers. However, it is only possible to interact with a single wavelength at one time using this technique.
A wedged etalon with an actuator that moves the etalon to the position of the channel to be added or dropped may also be employed. However, this system can only accommodate adding or dropping a single channel simultaneously. Further, the added or dropped channel must be at the same frequency.
Accordingly, in light of the limitations of the prior art, it is desirable to have an optical add/drop system which is simpler then those known in the art, has low optical loss characteristics, operates on single or multiple channels, and is capable of adding or dropping finely-spaced channels with separations as close as 50 MHz.
Optical Code Division Multiple Access. Code-Division Multiple Access (CDMA) is a spread spectrum encoding method that enables many users to simultaneously transmit separate signals over the same spectral bandwidth. In CDMA, a data signal of bandwidth D is modulated by a higher rate coded waveform of bandwidth C. The resulting signal has a bandwidth of D+C, which, for large ratios of C to D is approximately equal to C. The ratio C/D is commonly referred to as the spreading ratio, the spreading gain, or the processing gain. The intended receiver modulates the received signal by an exact replica of the coded waveform to remove the code modulation and recover the data signal. The coded waveform may be any of many types but the primary one of interest here is a binary coded bi-phase modulation, also referred to as binary phase shift keying, or BPSK, modulation. The signaling rate of the coded spreading waveform is commonly called the chip rate.
The number of users that could occupy the same spreading bandwidth C is regulated by the processing gain of the high rate modulation, i.e., the ratio of the modulation rate to the data rate, C/D. In theory, this ratio is equal to the number of users. But in practice, due to the need to maintain low cross-correlation properties between the high rate sequences, the number of usable sequences, hence users, is somewhat less than the processing gain.
There has been considerable interest within the communications industry in recent years on the potential for Optical CDMA (OCDMA) to make more efficient use of the bandwidth available in fiber optic communications systems. The main problem with fiber optic systems is the inefficient nature of dedicated bandwidth allocation architectures. Many communications, particularly Internet Protocol communications, are extremely bursty. Therefore, as more users are added and depart, the bandwidth resource must be dynamically re-allocated. This may not be feasible.
The traditional method of signal processing used to address this problem in fiber optic systems is a frequency domain multiplexing protocol called wavelength division multiplexing (WDM). In WDM, the optical efficiency is increased by the creation of a plurality of wavelengths, each carrying a separate signal. Still, the number of wavelengths or channels that can be supported is constrained by the stability of each discrete wavelength and the tuning range of the diode laser. OCDMA is suggested as an alternative or in conjunction with WDM to increase the efficient use of fiber communications systems. The primary advantage of code division multiple access, as opposed to other optical multiple access or multiplexing techniques, is the reduced requirement on coordination of exact timing and frequency allocations to the multiple users. In OCDMA, all of the users occupy the same time and frequency space and are precisely separated using their specific chipping code, a much simpler task.
Earlier inventions have been described to implement OCDMA, which can be grouped in three categories: simple spectral domain methods, complex spectral domain methods, and time-domain based systems.
In a spectrally encoded OCDMA system, each user is identified by a particular pattern of spectral (frequency) components. These patterns can be encoded with a simple periodic optical filter, as disclosed by Pfeiffer in U.S. Pat No. 5,784,506 and U.S. Pat. No. 6,215,573. In U.S. Pat. No. 5,784,506, Pfeiffer discloses an electronic decoding of the spectral encoded signal. In U.S. Pat. No. 6,215,573, Pfeiffer discloses an optical receiver with filtering characteristics for decreasing cross talk. In U.S. Pat. No. 5,867,290, Dutt et al. disclose a system whereby the spectrally encoded signal is created by selectively attenuating certain wavelengths from a broadband light source.
Typical OCDMA proposed systems use unipolar codes that use plus ones (+1) and zeros (0), generally called on-off keying. Such codes are used because they are easily optically detected. This inherently reduces optical efficiency because a “0” code removes or discards available light. To increase optical efficiency, it is far better to use bi-polar codes, i.e. those consisting of plus one (+1) and minus one (−1). However, detection of bi-polar codes requires detection in the presence of an unmodulated reference beam, i.e., coherent detection. Coherent detection is difficult and expensive to achieve in a practical system.
The Dutt system is not very optically efficient due to the use of unipolar codes and it has a limited code set. The Pfeiffer systems are more efficient but also have a limited code set resulting in a limited number of users.
In U.S. Pat. No. 5,760,941, Young et al. disclose a method and system for transmitting bi-polar codes using pairs of unipolar codes. This method requires each of the pairs of unipolar codes to be separately transmitted on separate fibers or opposite polarization.
In U.S. Pat. No. 6,236,483, Dutt et al. disclose a system based upon Young U.S. Pat. No. 5,760,941 with the addition of the use of sub-band encoding to divide the spectra into sub-groups.
Both the Dutt and Young systems are based upon attenuating the optical carrier using unipolar codes. This scheme cannot achieve optimal optical efficiency.
In U.S. Pat. No. 6,313,771, Munroe et al. disclose an OCDMA system based upon use of fiber Bragg gratings to encode a short pulse into a sequence of plus one (+1) and minus one (−1) coded pulses, i.e., optically efficient bi-polar codes. In order to overcome inherent limitations of fiber Bragg gratings, this method specifically uses two stages of encoding to achieve a relatively long encoding pattern. This multi-stage system is complex to build and relies on two fiber Bragg gratings. This is less optically efficient than using a single grating.
In U.S. Pat. No. 6,292,282, Mossberg et al. disclose a time wavelength multiple access communication system whereby the optical signal of a user is separated into a small number of spectral bands. The resulting bands are transmitted in a specific time-sequence order. A decoder for a specific user removes the time sequencing of the spectral bands such that a signal from the intended user is time-aligned. The number of frequency bands, and hence, the number of available codes, and therefore users, is limited in a practical system.
In summary, the existing methods of OCDMA are not very efficient which yields a lower number of potential users. Furthermore, they rely upon grating technologies that have limited resolution. Lastly, some of the more efficient methods are complex and costly to manufacture.
Wavelength Stabilization and Locking. Wavelength division multiplexing (WDM) systems communicate multiple signals through a single optical fiber by utilizing a different optical wavelength for each carrier signal. In the multiplexing process, an information signal is combined with a carrier signal and multiple such combined signals, called channels, are multiplexed into a single optical fiber for simultaneous transmission. De-multiplexing involves the separation of channels into individual data-carrying signals. The International Telecommunications Union has developed standards for WDM with predefined frequencies at channel spacings of 100 GHz (or 0.8 nm). By reducing the channel spacing, increased numbers of data-carrying channels may be added. Because of ever-increasing bandwidth requirements, telecommunications carriers need more channels of information and narrow channel spacings of 25 GHz and below are being intensely studied.
For a number of reasons, practical systems utilizing 25 GHz and narrower spacing are developing slowly. In particular, maintaining increasingly narrower channel spacing demands extreme precision in the frequency stability from the source laser—a precision that is not reliably achievable. The wavelength of most lasers has a tendency to drift, and if the channel spacing is sufficiently close, crosstalk is introduced as the wavelength of one channel drifts closer to an adjacent channel. Factors such as equipment aging, device tolerances, power source fluctuations, and temperature changes all serve to complicate the problem.
It is well known that the frequency stability of WDM systems is highly temperature dependent. Temperature changes cause variations in the optical devices that have a direct impact on optical properties, for example, by expanding or contracting a material to alter its physical dimensions or by changing the index of refraction of a material. The likely result is that the frequency of interest “drifts” relative to the target or detector with a corresponding degradation of the signal. Active compensation systems employ heater/coolers to maintain the components at a constant temperature. These devices effectively solve the problem of frequency drift, but at relatively high cost and with a loss of overall efficiency due to the power requirements.
As a result of this problem, prior art solutions have been found that attempt to eliminate or minimize temperature-induced frequency drift. The various alternative devices to which this type of solution is applied are commonly referred to as wavelength references, wavelength lockers, or wavelength monitors. These devices vary in size, complexity and cost. Among the best performing wavelength lockers, from the standpoint of size, accuracy and cost, are those utilizing etalons.
A well-known etalon-based optical device for performing wavelength locking is a Fabry-Perot etalon, an example of which was previously illustrated and described with respect to FIG. 1. When used in a wavelength locker, a portion of the modulated laser output beam is commonly split. One segment of the beam is routed directly to an output while a second segment first passes through the etalon before reaching a detector. Only the single wavelength λ exits the etalon and the device is designed to ensure that λ is the “lock” wavelength. Tuning is commonly achieved by physically rotating the etalon slightly during device fabrication. This position, and hence the lock wavelength of the device, is permanent once construction of the device is completed. Once the device is initially designed and calibrated, it defines a precise fixed relationship between the two signals provided to the detector. Variation of the relationship resulting from laser wavelength changes is monitored and the laser driver input is altered via a feedback loop to minimize detected differences. A more complete description of such a system may be found in the technical article entitled “Wavelength Lockers Make Fixed and Tunable Lasers Precise,” WDM Solutions, January 2002, p. 23.
The Fabry Perot etalon does not serve well as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a set of rings. Multiple wavelengths produce nested sets of concentric rings. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the etalon has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the beam can be fanned over a narrow range of angles to produce only a single-order output (e.g., m=+1) for each wavelength of interest. This fanning makes it easy to concentrate the output light at multiple detector points or fibers, but there is inherently high loss.
U.S. Pat. Nos. 5,428,700, 5,798,859 and 5,825,792 to Hall, Colbourne et al, and Villeneuve et al. respectively all reveal laser stabilization systems employing Fabry-Perot etalons of the type described above.
U.S. Pat. No. 6,345,059 to Flanders describes a highly complex laser wavelength compensation system in which the tuned wavelength is maintained by controlling the optical length of the laser cavity. This disclosure states that the wavelength precision of this system is 0.1 nm accuracy, which equals 12.5 GHz channel spacing.
U.S. Pat. No. 6,289,028 B1 to Munks, et al. discloses the simultaneous monitoring, stabilizing, tuning, and control of laser source wavelengths with the aid of an error feedback loop. A rotatable optical filter provides wavelength tuning by tilting the filter in accordance with feedback signals.
An alternative type of wavelength locking is taught in PCT application IPO Number WO 01/35505 A1 to Sappey. A one or two-dimensional array of lasers at different spatial positions within an external resonating cavity illuminates a diffraction grating. Opposing the diffraction grating is either a mirror (in the one-dimensional case) or a second grating (in the two-dimensional case). Light fed back to the lasers causes the laser to lock to the wavelength of the feedback, resulting in each laser lasing as a discrete, well-controlled wavelength. Each channel of a WDM system requires its own stabilized laser.
Significant channel spacing reductions in WDM systems will require substantial improvements in wavelength stability, with the corresponding precision ability to monitor, tune and lock those wavelengths as needed.
Flat-Top Filter Shape. A common identifier of signal filtering capability for a WDM device is called the passband. The shape of the passband determines the ability of the filter to efficiently separate channels. The most desirable passband shape, a horizontal top and vertical sides, would discriminate all of the light energy corresponding to a desired channel and would pass no other light energy from, for example, an adjacent channel. It is currently impossible to obtain such a filter in practice, but the more closely a device approximates this ideal the better its signal processing capability becomes. Prior art references are replete with references to “top hat” filters in reference to efforts to approximate the ideal filter shape.
Grating-based technologies are effectively limited to 50 GHz resolution. They commonly obtain a flat-top response by first widening and then flattening the Gaussian filter shape, as is described in U.S. Pat. No. 6,298,186 to He. An adverse effect of this technique is an approximate 3 dB loss in signal strength because of the abnormal shape of the resulting passband.
Polarization based devices are known to provide better peak flatness and isolation for narrow channel spacings than can be obtained with interferometric devices as is taught in U.S. Pat. No. 6,208,444 to Wong.
The present invention resolves many of the disadvantages in the prior art by providing more nearly ideal flat-top passbands at resolutions less than 50 GHz.
Interleaving and De-interleaving. An interleaver is an optical device in a wavelength division multiplexing fiber optic network that separates (de-interleaves) and combines (interleaves) alternating channels, i.e., even and odd channels. U.S. Pat. No. 6,275,322 describes a well-known architecture based upon a Michelson interferometer combined with a Fabry-Perot etalon. FIG. 10, when considered as having the input at Port A, illustrates an example of the functional operation of a typical de-interleaver. Multiplexed signals λ1, λ2, λ3, . . . enter interleaver 169 with a channel spacing of 100 GHz. Port B receives odd signals λ1, λ3, λ5, . . . with a channel spacing of 200 GHz. Port C receives even signals λ2, λ4, λ6, . . . , also with a channel spacing of 200 GHz. FIG. 10, when considered as having the inputs at Ports B and C, illustrates an example of the functional operation of a typical interleaver. Odd signals as described above enter at Port B synchronously with even signals entering at Port C. Interleaver 169 combines the signals to produce an output signal containing channels λ1, λ2, λ3, . . . with 100 GHz channel spacing. The channel spacing is always tied to the commercial telecommunications standard such as 100 GHz, 50 GHz, 25 GHz, etc., associated with DWDM signals. Interleavers work in this situation because even and odd channels have a frequency spacing that is double the frequency spacing for the combined set of channels. For example, in a 100 GHz spaced network, the odd channels are spaced by 200 GHz and the even channels are spaced at 200 GHz. By separating the even and odd channels, devices that are designed to work with 200 GHz spaced channels can interface with 100 GHz network devices.
By stacking interleavers, telecommunication carriers are able to improve bandwidth utilization and increase channel counts. Normally a fiber with channels spaced at 100 GHz carries 40 channels. Prior art FIG. 9 illustrates an example of how a carrier can increase this channel count by a factor of eight using seven interleavers. It is apparent from FIG. 9 that the prior art interleaver architecture is relatively complex and requires multiple devices, which introduces undesirable power losses into the system and increases device size and costs.
U.S. Pat. No. 6,275,322 B1 to Tai describes a prior art interleaver having a Michelson interferometer combined with a Fabry-Perot etalon. U.S. Pat. Nos. 6,212,313 B1 and 6,208,444 B1 describe optical interleavers that use the polarization property of light to separate odd and even channels. Their functional outputs are limited to two ports having one polarity or another.
There is a need for an optical interleaver suitable for the high data rate requirements of DWDM signal processing, that has the structural simplicity of a single device, and that does not have other disadvantages found within the prior art.
Optical Spectrum Analyzer. High-resolution spectrometers, or optical spectrum analyzers (OSAs), are used in a wide variety of optical applications. In particular, they are very important in fiber optic communication systems such as WDM where transmission power and laser performance is important to transmit information-carrying wavelengths. The higher the resolution of the OSA, the greater is the ability to monitor the fiber optic carrier.
Most all OSAs are based upon diffraction gratings. The gratings are many closely spaced grooves that scatter the incident light. The light scattered from each of the grooves creates an interference pattern some distance away from the grating that is used to obtain power estimates. In order to increase the ability to resolve the far-field pattern, the number of grooves must be increased as well as the spacing between the grooves must become more closely spaced. Although this can be accomplished, the size of the diffraction gratings and the expense of making such a large device creates a practical limit on the resolution of diffraction-grating based OSAs at 30 to 50 picometers. For example, U.S. Pat. No. 66,262,879 B1 claims a resolution of 4 GHz, which is equivalent to 32 picometers.
In a WDM system, the wavelengths of light being measured are on the order of 1.5 microns and they are spaced 50 to 100 GHz apart. Because of the high cost of laying optical fibers, achieving greater efficiency using existing fiber resources has become increasingly important. One trend in this direction is decreasing the spacing between wavelengths to 25 GHz, 12.5 GHz and less However, as channel spacing decreases common WDM network problems such as transmitter wavelength drift, filter concatenation, and signal distortion can more easily cause cross-talk between these closely-spaced channels. This makes it very important to continuously monitor all the channels in a functioning WDM system spaced at 25 GHz or less.
Integrated Optics. As with many technologies today, a principal goal of fiber optic telecommunication component manufacturing is to create WDM devices that are smaller, easier to fabricate, more reliable, and significantly cheaper than conventional products. To this end, multiplexer/demultiplexers, modulators, optical add/drop multiplexers (OADM), wavelength switches, and channel-monitoring equipment with integrated optical (IO) packaging and planar structures are increasingly populating the WDM component industry, including arrayed waveguide gratings (AWG) devices and planar lightwave circuits (PLC).
Several companies commercially produce integrated optic WDM devices, including Nortel, Lucent Technologies, JDS Uniphase, Corning, Bookham, and others, which are based on AWG and planar technologies. These devices perform a variety of functions within fiber optic networks and are becoming increasingly important as fiber optic systems and networks are required to handle larger volumes of data and voice traffic at faster rates. However, current WDM integrated optic technologies suffer from the same channel spacing and resolution limitations as their bulk optic counterparts. Wavelength separation closer than 50 GHz, within the ITU DWDM grid, has remained illusive to current planar and hybrid device technologies.
As is the case with their bulk counterparts, integrated optic (IO) WDM multiplexer and demultiplexer devices require optical elements to combine and separate the multiple wavelengths of the input signal. Many of the methods of wavelength separation used in bulk WDM demultiplexing equipment have also been incorporated into IO devices including diffraction gratings, AWGs, thin film filters, and Fabry-Perot interferometers. The operation, advantages, and disadvantages of these, and other dispersive methods, are addressed elsewhere in this patent application, and can be applied to integrated optical devices, as well.
A standard method of fabricating IO WDM demultiplexers/multiplexers devices is to place an AWG between a diffraction grating and a focusing lens. In such a configuration, the diffraction grating disperses the wavelengths, and lenses are used to couple discrete wavelengths into individual optical fiber. Examples of a planar 10 wavelength demultiplexers/multiplexer based upon this fundamental AWG structure are described in U.S. Pat. No. 5,799,118, in EP 0 984 304, and in U.S. Pat. No. 6,404,946. Although integrated optical diffraction grating-based devices, such as the aforementioned examples, are being used in WDM systems, their resolution is limited. An IO WDM device with a diffraction grating and an AWG would need to be very large and expensive to have sub-50 GHz channel resolution, and would suffer from very high loss.
An IO multi/demultiplexer that uses an echelon grating is described in U.S. Pat. No. 4,715,027, wherein the grating is formed on a dielectric substrate by photolithographic and etching techniques. In this case, the echelon grating could be made either wavelength-transmissive or reflective, depending on fabrication methods. This type of grating overcomes the size-dependency limitations and fabrication difficulties of other diffraction grating-based WDM devices. However, this device cannot produce the narrow channel spacing and high channel counts of an OTDL-based integrated optical demultiplexer.
In U.S. Pat. No. 4,279,464 Colombini describes and integrated optical wavelength demultiplexer that improves upon diffraction grating-based IO WDM demuxes by replacing the grating with a dispersive thin film prism within a multi-layer stack deposited onto a silicon wafer. By using a thin-film prismatic layer to angularly disperse and separate the wavelengths, the proposed device would have lower insertion losses and higher channel capacity. However, the fabrication of this IO demultiplexer could be extremely difficult and expensive due the precision required in the thin film deposition process to lay down the prism and lens layers, as well as the nature of the stack materials.
FIG. 57 shows a prior art a planar waveguide integrated optical multiplexer and demultiplexer device, as disclosed by Bhagavatula in U.S. Pat. No. 6,111,674. In this device, a multiple-wavelength input signal is demultiplexed using a Fabry-Perot thin film stack consisting of alternating partially reflective and transmissive layers. The angularly dispersed wavelengths emerge from the “optical path length difference generator” and are individually coupled to a fan-shaped output array of waveguides by means of a focusing lens. This device could be fabricated as either a planar or a Hybrid IO structure. The drawback to this type of integrated optical WDM demultiplexer/multiplexer is the inherently high loss associated with the thin film wavelength-separation elements, which limits the number of channels that can be effectively demultiplexed, as well as the level of channel spacing within a DWDM system.
The OTDL technology and devices described herein allow for fabrication of WDM products with much finer wavelength channel spacing than currently possible with optical communications equipment, by operating in very narrow bands within each 100 GHz ITU DWDM channel. One can achieve data-carrying channels that are as narrow as 2.5 Hz per bit without pulse shaping, and can reach 0.8 Hz per bit with pulse shaping using OTDL technology. In practical application, this means that an OTDL device can carry sixteen OC-48 (2.5 Gbits/sec) data channels over each 100 GHz DWDM channel.
One approach for fabrication of OTDL devices is a bulk optical approach composed of discrete components with free-space optical connections, as disclosed herein and in co-pending U.S. patent application Ser. No. 09/687,029. Such a method of construction could be very labor intensive and expensive as each optical component and sub-system must be aligned and fixed in place. Maintaining important fabrication and functional tolerances can be difficult. In addition, as functionality of the bulk OTDL device is increased, so are the part count and device size. In today's WDM networks, this can be a disadvantage as component dimensions are crucial to system construction and cost.
The fundamental operation of the OTDL is similar to that of AWG technology, based on performing a Fourier Transformation of a set of time-offset signal samples. However, AWG technology relies on a set of individual waveguides, each of a different length, to create the time-offset samples, while the OTDL technology uses an optical cavity with a partially transmissive surface as an optical delay line. This method allows the creation of a larger array (more elements) of time-offset samples, and greater time delay between samples, leading to higher resolution and an increased channel count. Furthermore, the optical cavity technique affords a smaller package footprint than that of an AWG device performing an equivalent function.